Functional and sequence characterization of agkicetin, a new glycoprotein IB antagonist isolated from agkistrodon acutus venom

A new glycoprotein Ib (GPIb) antagonist, agkicetin, was purified from the venom of Agkistrodon acutus and characterized. It is a disulfide-linked heterodimer consisting subunits of 15 and 14 kDa. The subunits are homologous to each other and to other snake venom proteins of the C-type (Ca²⁺-dependent) lectin superfamily. Agkicetin behaved as a potent antagonist of von Willebrand Factor (vWF)-induced platelet agglutination (IC₅₀=12.5 nM) and bound specifically to GPIb of fixed platelets with high affinity (K_d=38 nM). It did not bind coagulation factor IX and thrombin. Monoclonal antibody against epitope on the N-terminal domain of GPIb competed the binding of agkicetin with platelets. Reduced and alkylated agkicetin lost most of its inhibitory efficacy toward vWF-induced platelet agglutination.